In the field of telecommunications, optical networking is a well established technology for transporting data at a high bit rate over long distances. The basic components of optical networks are an optical transmitter (such as a laser transmission system) for generating and modulating an optical signal, an optical waveguide (such as a fiber optic cable) for transporting the optical signal, and a photodetecter for receiving the modulated signal and converting the optical signal into an analogous electrical signal. While fiber-doped amplifiers well known in the art have been used to extend a distance over which the optical modulated signal can be conveyed, and numerous passive optical components have been developed to permit switching, and signal control within the optical domain, the requirement of converting the optical signal into an electrical format for digital computing, switching, regenerating, reformatting, etc., has remained an essential part of this technology.
Numerous different devices are known for generating analog electrical signals from optical signals. Semiconductor diodes that exhibit photoconductivity have been found to be particularly efficacious. Photoconductivity is a property of some diodes (photodiodes) that carry an electric current between an anode and a cathode, at a rate proportional to electromagnetic energy incident a depletion region disposed between the anode and cathode. Some (variable gain) photodiodes provide a means for variably magnifying a received optical signal to produce an analog electrical signal effectively multiplied by a scaling factor, or “gain”. The gain of a variable gain photodiode is applied in relation to a physical parameter of the device to control a sensitivity of the photodiode to receive light.
Variable gain photodiodes are therefore very useful for receiving an optical signal that is attenuated in transit through the waveguide when the attenuation varies with one or more factors that are not directly controlled. For example, avalanche photodiodes (APDs) provide variable gain in dependence on a reverse voltage bias across the depletion region, and a consequent amount of impact ionization produced by the photons impinging on a crystalline lattice within the depletion region. Naturally it is important to control the gain of APDs in order to provide adaptive operating magnification of the analog electrical signal in response to an optical power of the received signal, a temperature of the photodiode, and any other relevant parameters. To this end U.S. Pat. No. 6,313,459, which issued on Nov. 6, 2001 to Hoffe et al., is informative and is incorporated herein by reference.
While APDs are of well recognized utility, an intrinsic limit on the current that can be transported across the depletion region poses problems because of an absence of any equivalent limitation on impact ionization. The charges produced by the electromagnetic energy incident on the crystalline lattice accelerated by the voltage bias are not regulated by anything more than the optical power, and the voltage bias, but if conduction across the photodiode is saturated, these charges have no path to ground, which may result in excessive heat, melting of affected regions, and damage to the photodiode. It is therefore necessary to prevent optical overloading of the photodiode with respect to the operating gain.
It is known in the art to control optical intensities of respective channels in wavelength division multiplexed (WDM) optical data transport systems using variable optical amplifiers. For example, a prior art optical data transport system shown in FIG. 1 schematically illustrates two network elements (NEs) 10a,b of such a data transport system. The NE 10a includes a transmitter 12 having a laser and modulation system 14 for launching a modulated optical signal. The transmitter 12 includes modulation control firmware 16 for driving the laser and modulation system 14, to impress the desired data onto a wavelength carrier to which the laser is tuned. A number of the optical wavelength carriers are produced in a like fashion, and a variable optical attenuator (VOA) 18 is applied to respective channels, in order to ensure substantially equal mean optical powers of each of the wavelength channels. This constant mean optical power is usually preferred because of an affect of doped-fiber amplifiers on imbalanced wavelength channels. The VOAs 18 are capable of independently attenuating the wavelength channels in response to downstream feedback, and may, in effect, reduce optical overload at downstream receivers.
The NE 10b includes a receiver 20 for receiving and demodulating the optical signal transmitted by the NE 10a. The receiver 20 includes a variable gain photodiode 22 that converts the optical signal into the analog electrical signal. Firmware of the receiver 20 is provisioned with program instructions to effect a negative feedback control loop 24 to control a sensitivity of the variable gain photodiode 22, in a manner that is somewhat analogous to automatic gain control circuits well known in the art, although automatic gain control is generally applied to a pre-amplifier receiving an analog electrical signal and so is entirely in the electrical domain. More precisely, the feedback control loop 24 reduces the sensitivity of the photodiode when a mean optical power of the received signal rises (over a predetermined interval), and increases the sensitivity if the mean optical power subsides.
While each NE typically includes at least one shelf of numerous receiver and transmitter cards, in the illustrated embodiment only one such transmitter 12 and receiver 20 is shown. For example, certain known NEs include a shelf having sixteen data receiver and transmission cards, a shelf card, and two switch control cards. Optical wavelength division multiplexing of a plurality of optical signals generated by numerous transmitters 12 may be performed by a multiplexer 26, and the multiplexed optical signal may be transmitted over an optical fiber waveguide 28. In such cases, a demultiplexer 30 is used to extract from the multiplexed optical signal, the wavelength carriers that are modulated to form the optical signal. The optical fiber waveguide 28 may span hundreds of kilometers and may include any number of optical components such as erbium doped fiber amplifiers well known in the art. In some embodiments, a sensor 32 taps off a predefined proportion of the multiplexed optical signal, and analyzes the power spectrum produced in order to compute a control signal used for controlling the independent wavelength channels at the VOA 18. In other embodiments the control signal is produced by the receiver 20 in response to a power, bit error rate, or quality of the received optical signal. The effect of the control of the VOA 18 may reduce an occurrence of overload optical power impinging on the photodiode 22, but it is an expensive solution, and does not permit the explicit identification of conditions that are likely to contribute to overload optical signals so that damaging optical signals can be reduced.
A Patent Abstract of Japan publication number 01157632 that names Murakami Taisuke as inventor, teaches a receiver having an optical attenuator that is controlled to reduce optical power of a modulated optical signal incident on a photodiode, in order to maximize a dynamic range of the photodiode. It will be appreciated by those skilled in the art that controllable variable optical attenuators are expensive, and that any attenuation of the optical signal constitutes a debit of a “link budget”, because attenuation comes at a cost of a maximum distance between the laser and the receiver within which reception of the optical signal is consistently satisfactory.
There therefore remains a need for a method and apparatus for preventing damaging optical overload of a variable gain photodiode in an optical receiver.